In Release 8 (hereinafter, referred to Rel. 8) of the 3rd Generation Partnership Project Radio Access Network Long Term Evolution (3GPP-LTE, hereinafter, referred to as “LTE”), orthogonal frequency division multiple access (OFDMA) is employed as a downlink communication scheme, and single carrier frequency division multiple access (SC-FDMA) is employed as an uplink communication scheme.
In the downlink of Rel. 8, a cell specific reference signal (hereinafter, referred to as CRS) is used as a reference signal for demodulation of a data signal (PDSCH). The CRS is a reference signal commonly used within a cell. The CRS is transmitted in a time/frequency resource depending on a cell ID and is transmitted to cover the entire area of the cell. Further, the CRS is transmitted in all subframes. Further, the CRS is also used for measurement for link adoption and mobility management such as cell selection. That is, a terminal (called user equipment (UE)) measures reception power (reference signal reception power (RSRP)) or reception quality (reference signal reception quality (RSRQ)) using a CRS of a cell (serving cell) to which the UE is connected and a CRS of an adjacent cell. Further, when a predetermined reference is satisfied, for example, when the RSRP of the adjacent cell is higher than the RSRP of the serving cell by 3 [dB], the terminal reports the cell ID and the RSRP of the adjacent cell. In this case, when information relating to the cell ID of the adjacent cell is broadcasted, the terminal may perform cell detection using the cell ID (for example, see NPL 1).
In contrast, LTE-Advanced (hereinafter, referred to as “LTE-A” or “Release 10 (Rel. 10),” which is an advanced version of LTE (Rel. 8) supports data transmission using a demodulation reference signal called “DMRS” or “UE specific Reference Signal” as an extension of multiple input multiple output (MIMO) transmission in the downlink. While the CRS is transmitted to the entire cell, the DMRS is transmitted to a terminal for which data is assigned, so that the DMRS enables beam formation by precoding, and data transmission of high throughput (for example, see NPL 2, NPL 3 and NPL 4). In Rel. 10, the transmission using the DMRS can be used for a terminal in which transmission mode 9 is set.
Further, channel state information (CSI) used for link adoption or scheduling is measured using a CSI-RS. The CSI-RS is transmitted in different resources (time, frequency or code) from respective antennas (antenna ports). For example, the CSI-RS is normally transmitted at an interval of about 10 subframes (10 ms). Further, resource information on the CSI-RS that is to be a CSI measurement and reporting target in a terminal is indicated to the terminal (UE) from a base station (or referred to as “eNB”). The CSI includes a channel quality indicator (CQI) indicating reception quality (SINR) or achievable data rate, and a precoding matrix indicator (PMI) indicating an optimal precoding matrix.
Further, in Release 11 (hereinafter, referred to as Rel. 11), which is the next release of Rel. 10, studies have been carried out on coordinated multi point transmission and reception (CoMP) in a heterogeneous network that uses a plurality of base stations having coverage areas different in size. The heterogeneous network is a network that includes a macro base station (high power node (HPN)) and a pico base station (low power node (LPN)) or a remote radio head (RRH). In the CoMP network, a plurality of nodes (transmission points (TP)) perform data transmission and reception with a terminal in a coordinated manner. Here, in the system of the related art, each transmission point forms a cell having a different cell ID. Accordingly, a CRS transmitted in a time/frequency resource arrangement depending on the cell ID is transmitted in a different arrangement for each cell. For this reason, data (PDSCH) is also transmitted in a different resource arrangement, so that coordinated transmission from the plurality of transmission points is performed in a restricted manner.
In this respect, in Rel. 11, a CoMP operation using the same cell ID has been discussed (for example, see NPL 5). The CoMP operation using the same cell ID refers to an operation in which the same cell ID as the cell ID of an HPN (macro base station, Macro eNB) is assigned to a plurality of LPNs (pico base stations) in a macro cell (cell covered by the HPN) (for example, see FIG. 1). In such an operation, since the cell IDs of the HPN and the LPN (hereinafter, referred to as transmission points) in the same macro cell ID are the same, CRSs transmitted in a resource depending on the cell ID are transmitted in the same resource at the plurality of transmission points. Accordingly, the CRSs transmitted from the respective transmission points are combined in a manner for signals transmitted in single frequency network for reception in a terminal. Further, a data channel (PDSCH) and a control channel (PDCCH) for each terminal to be demodulated using the CRS are transmitted in the same time/frequency resource from all the transmission points in order to maintain the same phase relationship with the CRS. Thus, in Rel. 11, a method for transmitting a PDSCH for a different terminal using the same time/frequency resource from different transmission points using a UE specific reference signal (for example, DMRS) has been discussed.